Paper separators consisting mainly of solvent spun cellulose or recycled cellulose fibers have conventionally been used as separators of electric double layer capacitors (see, for example, Patent Documents 1 to 3). Although electrodes composed of activated carbon have conventionally been used for the electrodes of electric double layer capacitors, their energy density and power density were inadequate for applications such as auxiliary power supplies of automobiles and railroad vehicles. Since energy density and power density increase in proportion to the square of the voltage, there has been considerable activity in the improvement of electrode materials. While the upper limit voltage of conventional activated carbon electrodes is about 2.5 to 2.7 V, an electrode operating at an upper limit voltage of about 3.7 to 4.2 V has been developed recently.
When paper separators are exposed to a high voltage of at least 3 V, oxidative deterioration occurs resulting in a considerable decrease in strength possibly leading to tearing of the paper. Thus, paper separators are unsuitable for use as separators of electric double layer capacitors operating at such high voltages. The inventors of the present invention have disclosed a capacitor separator comprising a wet nonwoven fabric which contains fibrillated liquid crystal polymer fibers, non-fibrillated organic fibers having a fineness of 0.5 dtex or less, and non-fibrillated organic fibers having a fineness of 1 to 3.3 dtex (see Patent Document 4). However, further improvement is desired with respect to leakage current, internal resistance, capacitance fluctuations, capacitance retention rate and vibration resistance. A separator for an electric double layer capacitor has been disclosed that is comprised of a fiber sheet containing fibrillated organic fibers and narrow polyester fibers having a fineness of 0.45 dtex or less (see Patent Document 5). However, this separator had problems with internal resistance, leakage current, capacitance retention rate and vibration resistance. Here, vibration resistance refers to a property required in the case that separators are mounted in an automobile or railroad vehicle. Separators are required that are not damaged or do not allow the formation of holes due to vibrations generated during idling or moving of automobiles or railroad vehicles.
[Patent Document 1] Japanese Unexamined Patent Publication No. H5-267103
[Patent Document 2] Japanese Unexamined Patent Publication No. H11-168033
[Patent Document 3] Japanese Unexamined Patent Publication No. 2000-3834
[Patent Document 4] Japanese Unexamined Patent Publication No. 2003-124065
[Patent Document 5] Japanese Unexamined Patent Publication No. 2001-244150